1. Field of the Invention
The present invention relates to an apparatus and method for spraying adhesive in a selected pattern on a continuously moving web. The invention more particularly relates to an apparatus and method for accurately spraying a predetermined volumetric flow of adhesive on a continuously moving web in a selected pattern.
2. Description of the Related Art
Absorbent articles, such as disposable diapers, training pants, adult incontinence articles and the like, generally include several different components which are adhesively bonded together. For example, adhesive has been used to bond individual layers of the absorbent article, such as the outer cover and bodyside liner, together. Adhesive has also been used to bond discrete pieces, such as the fasteners and leg elastics, to the article. Typically, the adhesive has been sprayed or slot-coated on the continuous moving web which provides the absorbent articles. The sufficiency of the adhesive bond between the components of the absorbent article is generally dependent upon the amount of adhesive, the type of adhesive and the spray pattern of the adhesive.
Various techniques for spraying an adhesive on a moving web are well known to those skilled in the art. Many conventional techniques have relied upon pressure to deliver the adhesive to a plurality of nozzles and spray the adhesive from the nozzles on to a moving web. In such conventional techniques, the amount and pattern of the spray of adhesive is directly dependent upon the pressure at each nozzle which can vary depending upon many factors including the pressure at which the adhesive is supplied at, the pressure losses within the system, the number of nozzles which are operating at a given point in time and whether the nozzles are obstructed in any manner. For example, one or more nozzles have frequently become either partially or totally blocked thereby inhibiting the flow of adhesive through the nozzle. As a result, in such a pressure-dependent system, the pressure at each nozzle and the subsequent flow rate of adhesive from each nozzle has been difficult to control and predict.
For example, one conventional technique which relies upon pressure for spraying the adhesive involves an apparatus having a plurality of nozzles which are connected to a manifold. Adhesive is supplied to the manifold and nozzles by a single large, central tank of molten adhesive. The tank of adhesive is pressurized to deliver the adhesive from the tank to the manifold and to the individual nozzles. The individual nozzles are then independently turned on and off to spray the adhesive. The amount of adhesive which is dispensed from the nozzles is dependent upon the pressure at the tank of adhesive, the length of the supply lines, the number of nozzles which are being operated at a given point in time and whether the nozzle are operating efficiently.
Another conventional technique which relies upon pressure for spraying adhesive is similar to the above technique except the adhesive is continuously supplied from the central tank to the manifold and nozzles by a pump. The pump is configured to continuously supply a flow of adhesive to the manifold. The manifold includes a means for recirculating the adhesive such that any unused adhesive may be recirculated back to the central tank of adhesive through a recirculation line. The pump generally runs continuously whether or not adhesive is being sprayed from the nozzles.
In such a configuration, the manifold includes a pair of valves for each nozzle to control the flow of adhesive to the respective nozzle. A first valve is operated to control the flow of adhesive to the recirculation line leading back to the tank of adhesive and a second valve is operated to control the flow of adhesive through the nozzle. The first valves in such a system may be pressure activated valves which will automatically open to recirculate the adhesive when a certain pressure is reached. In operation, the first valve is closed and the second valve is opened to allow adhesive to flow through the nozzle to spray the adhesive. The nozzles in the manifold may be operated independently, all together, or in selected groups. In such a configuration, the amount of adhesive which is dispensed from each nozzle depends upon the amount of adhesive which is supplied to the manifold, the number of nozzles which are being operated at a given point in time, the amount of adhesive being recirculated and whether the nozzles are operating efficiently.
Still another conventional technique for spraying adhesive is similar to the above techniques except that the recirculation line which allows adhesive to flow back to the supply tank is continuously maintained in an open position. In such a configuration, the amount of adhesive which is dispensed from the nozzles is again dependent upon the pressure variations in the system.
Other conventional techniques for spraying an adhesive on a moving web have relied upon volumetric displacement to deliver and spray the adhesive from a plurality of nozzles. In a volumetric displacement system, a predetermined volumetric flow rate of adhesive is delivered to each nozzle. One conventional technique which relies upon volumetric displacement for spraying the adhesive involves an apparatus having a plurality of nozzles. Adhesive is supplied to each nozzle by it's own pump which is connected to a single large, central tank of molten adhesive by a supply line. Each individual nozzle/pump combination includes a pair of valves that control the flow of adhesive. For example, one valve is positioned in the supply line leading from the tank of adhesive to the individual pump, and a second valve is positioned in the supply line leading from the pump to the respective nozzle. The pumps generally run continuously whether or not adhesive is being supplied from the tank. The nozzles may be operated independently, all together, or in selected groups. In operation, the valve between the tank and the pump is opened to allow adhesive to flow to the pump, and then the valve between the pump and the nozzle is opened to spray the adhesive. Another conventional technique which relies upon volumetric displacement for spraying adhesive is similar to the above technique except that each nozzle is capable of being operated between an on position and an off position and each nozzle/pump combination includes a mechanism for recirculating the adhesive back to the tank of adhesive when the nozzle is operated in the off position. For example, a valve may be positioned in a recirculation line which is connected to the adhesive supply line extending between the individual pump and the respective nozzle. In operation, adhesive is continuously supplied from the pump to the nozzle. To spray the adhesive, the valve in the recirculation line is closed and the nozzle is subsequently turned to an on position. To stop the spray of adhesive, the nozzle is turned to an off position and the valve is subsequently opened to recirculate the adhesive.
Many of the conventional techniques for spraying adhesive, such as those described above have not been completely satisfactory. For example, in techniques which utilize pressure to supply adhesive to a manifold which includes a plurality of nozzles, it has been difficult to regulate the amount of adhesive which is sprayed from each individual nozzle as each nozzle is independently turned on and off. Typically, the adhesive has been supplied to the manifold at a constant pressure. As such, the mass flow rate of the adhesive through each nozzle has undesirably fluctuated as the number of operating nozzles has varied and the recirculation line has been opened and closed. For example, as the number of operating nozzles is decreased, the mass flow rate of the adhesive to the remaining nozzles will be increased due to the constant pressure applied to the manifold. As a result, the amount of adhesive sprayed from each nozzle and the corresponding pattern of the spray has undesirably varied as the number of operating nozzles has varied. In addition, the nozzle have frequently become either partially or totally obstructed which has also undesirably varied the amount of adhesive being sprayed from each nozzle.
Moreover, in the conventional techniques which have utilized a pair of valves to control the flow of adhesive, it has been difficult to provide an instantaneous spray of adhesive at the desired amount. For example, since both valves are required to be operated to supply adhesive to the nozzle, the adhesive has not been immediately supplied to the nozzle at the desired flow rate to provide the desired spray pattern of adhesive. Typically, the spray of adhesive from each nozzle as it is turned on has been less than the desired amount and the desired amount has not been reached until a period of time has lapsed. Such inconsistent amounts of adhesive have led to insufficient adhesion between the webs being bonded together. Control mechanisms have been utilized to provide a time delay between the opening of the valves in an attempt to provide the requisite flow of adhesive to the nozzle before it is opened. However, it has been difficult to accurately time the valves to instantaneously provide the desired pattern of adhesive at the desired amount. This problem has been particularly acute when attempting to intermittently spray the adhesive on a continuously moving web in a controlled, intermittent pattern.
The above-mentioned problems encountered in spraying an adhesive have been particularly evident and difficult to solve when compared to spraying other liquids, such as water, inks and the like, due to the relatively high viscosity of adhesives. Moreover, typical adhesive applications require a relatively low mass flow rate of adhesive which makes it increasingly difficult to regulate the amount of adhesive being sprayed on the moving web.
As a result, it remains desirable to provide an apparatus and method which are configured to instantaneously and accurately control the amount and spray pattern of an adhesive on a continuously moving web. In particular, it remains desirable to provide an adhesive spraying system which is capable of accurately and reliably spraying a predetermined volumetric flow of adhesive on a continuously moving web. In addition, it remains desirable to provide a plurality of nozzles, each of which is adapted to spray an independent, predetermined volumetric flow of adhesive and provide an almost instantaneous spray of adhesive in the desired amount and pattern as the nozzle is operated from an off position to an on position.